1. Field of the Invention
The present invention relates to a measuring device comprising an ISFET used as a chemically selective ion sensor, a reference electrode, an amplifier and a control/correction circuit. The device is operable to maintain the drain-source current I.sub.DS of the ISFET at a constant value.
2. Description of the Prior Art
It is known that drift effects in ISFET sensors, such as Si.sub.3 N.sub.4 and Al.sub.2 O.sub.3 gate ISFET sensors, are highly detrimental to the accuracy and stability of measuring systems including such ISFET sensors. These drift effects restrict the applicability of such ISFET sensors, especially in the medical field.
A further drawback inherent in ISFET sensors is the long period of adjustment required when commencing a measuring procedure. As a result of such long period, acceptably stable measurements can only be performed after this period has elapsed.
The degree of accuracy required for a biomedical pH-sensor is very high and should be at least 0.03 pH within a measuring period of a number of hours, and such accuracy preferably should be attained at the beginning of the measuring period. Many present-day ISFET sensors fail to fully meet these requirements as to accuracy.
For example, an ISFET sensor having a pH membrane/gate made of Al.sub.2 O.sub.3 or Si.sub.3 N.sub.4 appears to have a drift of approximately 0.02-0.06 pH/hour, and the initial drift is still far higher and may even reach values of 0.1-0.2 pH/hour during the first hour of operation of the ISFET sensor. Accordingly, such presently available ISFET sensors are not suitable for use in the biomedical field.
As will be described in greater detail hereinafter the measuring device of the present invention provides a solution to the problems of drift described above.